1. Field of the Invention
The present invention relates to a light source system for an endoscope. More particularly, the present invention pertains to a light source system for an endoscope which is designed so that it is possible to adjust the quantity of illumination light which is applied to the incident end of a light guide from a light source.
2. Description of the Related Art
The type of light source system for an endoscope is generally arranged such that illumination light which is emitted from a light source lamp is formed into parallel rays by means of a reflecting mirror and these parallel rays converge through a condenser lens so as to enter the incident end of an illumination light guide. A movable light-shielding plate which partially intercepts the illumination light is provided at an intermediate position along the path of the illumination light which travels from the light source lamp toward the incident end of the light guide. The light-shielding plate can be sidewardly advanced into and withdrawn from the illumination light path, and the position of the movable light-shielding plate is automatically controlled to adjust the quantity of illumination light.
The above-described prior art suffers, however, from the following problems. Since the illumination light is partially intercepted by means of the movable light-shielding plate which is sidewardly advanced into the illumination light path, as the quantity of illumination light is decreased, the cross-sectional area of the illumination light path gradually narrows down and eventually only the illumination light which passes through the peripheral portion of the illumination light path enters the light guide. Since the illumination light is refracted to a substantial degree at the peripheral portion of the lens, the optical paths of components of the refracted light diverge from each other in accordance with the wavelength of each light component, which results in a change in the spectral characteristics of the illumination light entering the light guide. As a result, no precise color reproduction is available for an observed image on a monitor screen. Further, since in the prior art, a delay in the response of the movable light-shielding plate is present, the brightness of the observed image on the monitor screen cannot be maintained at a constant level.